In a mobile communication system such as a portable telephone set (for example, Personal Handyphone System: hereinafter abbreviated as PHS) rapidly developing in recent years, synchronization detection according to the so-called correlation synchronization is generally known as a method of finding out a synchronous position of a signal received from a mobile terminal unit (Personal Station: hereinafter abbreviated as PS) in a base station (Cell Station: hereinafter abbreviated as CS).
Generally known synchronous position detection according to correlation synchronization is now schematically described.
When a CS generally receives a signal from a PS in time division multiplexing, the CS cannot demodulate the received signal to data unless specifying a position where an information part of the signal starts (start symbol in a PHS: hereinafter abbreviated as SS) on an absolute time axis excluding an initial unnecessary component. This operation of specifying the start position of the information (specifying the time position of the SS in the PHS) is generally referred to as detection of a synchronous position.
The idea of correlation synchronization described below is widely employed as such a method of detecting the synchronous position. In consideration of that the signal received from the PS includes a known reference signal common to all users, a known waveform of this reference signal is stored in a memory according to this method. A combination of a preamble and a unique word is employed as the reference signal in the PHS, for example, and the preamble is formed by a bit string of repetition of “1001” while the unique word is formed by a prescribed bit string varying with transmission from the PS side and transmission from the CS side.
The waveform of the received signal of a constant length is sliced with displacement, for calculating correlation between the same and the waveform of the reference signal stored in the memory. A time position where the calculated correlation value peaks is specified as the synchronous position.
The process of this specification of the synchronous position is described with reference to a waveform diagram of FIG. 11. Referring to FIG. 11, (a) shows the waveform of the known reference signal stored in the memory of the CS. On the other hand, (b) in FIG. 11 shows the waveform of the signal continuously received in the CS from the PS.
When signal components are sliced from the received signal (b) with displacement by a time length equal to the time length of the reference signal (a) and the sliced waveform is compared with the waveform of the reference signal, the correlation value of these signals increases (1.0 when completely matching) if the waveforms match with each other while the correlation value lowers if the waveforms mismatch with each other. Referring to FIG. 11, (c) shows a correlation function consisting of such a correlation value of the reference signal (a) and the received signal (b).
In the example shown in FIG. 11, the waveform of the received signal (b) on the time axis matches with the fixed waveform of the reference signal (a) in sections specified by broken lines {circle around (1)} and {circle around (2)}. Therefore, it follows that the correlation value of the waveforms peaks on a time position (shown by a broken line {circle around (3)}) implementing such matching of the waveforms, while the correlation value lowers (approaches zero) in other time positions.
Thus, it is possible to specify the synchronous position on the time axis by continuously calculating the correlation value of the fixed waveform stored in the memory of the CS and the partial waveform of the received signal and finding out the peak value thereof. The above is the basic idea of conventional correlation synchronization.
The aforementioned description of correlation synchronization is premised on that a signal from a single PS is independently received in the CS. However, the CS may receive signals of the same frequency from two PSs in an overlapping manner.
In path division multiple access (hereinafter abbreviated as PDMA) spatially dividing a single time slot of the same frequency and allocating channels to a plurality of PSs, for example, two signals of the same frequency may arrive at the CS in an overlapping manner.
Regardless of such a PDMA system, further, the CS may receive a radio wave from another PS connected with a CS in another cell as an unnecessary interference radio wave in a state overlapping with a signal of the same frequency from a desired PS.
In this case, the synchronous position of the signal from the desired PS cannot be specified by the aforementioned method of correlation synchronization. FIG. 12 is a waveform diagram for illustrating such a state that the synchronous position is hard to specify.
Referring to FIG. 12, each of (a) to (c) is a graph showing change (correlation function) of the correlation value between the fixed waveform ((a) in FIG. 11) and received waveforms in the case where two signals of the same frequency are received in the CS in an overlapping manner.
When two signals of the same frequency are received in an overlapping manner, two peak values may appear on the correlation value as shown in FIG. 12(a), to disable identification of the signal received from the desired PS.
As shown at (b) in FIG. 12, only a single peak value may appear between two synchronous positions where two peak values must originally appear, in place of the two peak values.
When the two received signals are remarkably different in signal power from each other, the synchronous position of the signal having weaker power may be overlooked as shown at (c) in FIG. 12.
As hereinabove described, the conventional method of correlation synchronization has such a problem that the synchronous position of the desired PS cannot be specified when the CS receives two signals of the same frequency in an overlapping manner.
Accordingly, an object of the present invention is to provide a radio receiving system and a synchronization detection method capable of detecting the synchronous position of a signal received from a desired PS and correctly extracting the received signal by a method of correlation synchronization even when a CS receives signals of the same frequency in an overlapping manner.